The pushpieces currently employed in connection with mechanical timepieces always act on a mechanism either to change an indication or to start or stop a special function, for instance a counting function (chronograph). Through the operation of a mechanism which passes from one state to another, there results a discontinuity or a brusque variation in the force exerted on the pushpiece by the user, this being manifested by a click generally audible accompanied by a clear perception to the touch of the variation of the force exerted. The user thus knows without it being necessary to make a visual inspection, that he was effectively brought about the concerned operation.
In the general run of electronic timepieces the functions are taken up by electronic circuits and their operation is brought about by means of simple contact switches controlled by the user by means of pushpieces. The previously employed mechanisms having become superfluous, the setting of the function is no longer determined with certainty by the user. He may thus experience doubts as to the effectiveness of the operation and has no other possibility than that of a visual inspection and even this possibility does not always exist. Such situation is further aggravated by any hesitation or trembling in the hand of the user. If he determines that his effort has not given rise to an effect, it is necessary to repeat it and again check the result. If his first control gesture were synchronized with an event serving as a point of departure for instance of a counting function, the effective operation does not correspond to this event and there will be an uncertainty as to the result of the measurement, such uncertainty depending on the reaction time of the user. This uncertainty is even more troublesome if, as is now very well known, one should wish to measure a time interval to the hundredth of a second, this being possible with electronic means without any intrinsic difficulties.
Attempts have been made to overcome these difficulties. For instance, Swiss Pat. No. 629.647 (US Pat. No. 4,451,719) discloses a pushpiece comprising a stem sliding in a tube against the return force of a spring in which pushpiece an elastic element is interposed between the stem and the tube. This element is axially retained in an accommodation arranged in the stem and cooperates with a surface borne by the tube, such surface exhibiting a sufficient variation in its slope in order that a click or decoupling is produced when the elastic member crosses over the change of slope. There results from what has just been said that the entire click mechanism is incorporated in the pushpiece, such complicating its construction and increasing its dimensions, above all in diameter, thus causing it to be poorly adapted to a thin chronograph.
Another attempt at overcoming the difficulties which have just been mentioned incorporates the click mechanism directly in the movement of the timepiece, such mechanism being then controlled by a pushpiece reduced to its most simple form. Such a construction is disclosed in the Japanese utility model JP No. 7 812/79 published Apr. 11, 1979. In this construction, a simple pushpiece acts on a spring mounted on the base plate of a timepiece movement. The spring maintains the pushpiece in its rest position. This spring includes a portion serving to fasten it to the base plate, an elastic portion, a portion in U form and finally an engaging portion serving to cooperate with a stud driven into the base plate. When the pushpiece passes from the rest position to the pushed-in position, the engaging portion butts initially against the stud, then climbs thereon, thus producing the click or decoupling effect sought after. This construction, in order to operate correctly, requires a very precise positioning of the stud driven into the base plate relative to the fixation point of the spring on said base plate. In this embodiment, the inevitable manufacturing tolerances will have as a result that the force applied to the pushpiece will vary over large limits as a function of such tolerances. Not only must the spring be cut out with precision, but further it must cooperate with fixed points which must be positioned with a tolerance which does not depend on the cutting of the spring, but on the implantation of such points fixed with reference ot the base plate. It will be likewise understood that in cases where the force of the disengagement is very high, the wear of the stud may be very rapid, which will bring about a diminution of such forces, indeed a shortened life of the pushpiece mechanism.